The present invention relates to a device for calibrating an apparatus for measuring the formation index of a sheet of paper. It relates, in particular, to a device for calibrating an apparatus for continuously measuring the characteristics of a sheet of paper, said apparatus being commonly referred to by a person skilled in the art as a "formation sensor".
In the paper-making industry, the concept of formation involves the regular distribution and amount of the fibrous material in the plane of a sheet of paper which, together with the basis weight components, determine the characteristics of the paper. In the above patent, Applicant described a formation sensor comprising fundamentally a light source directing a polarized laser beam through the sheet of paper to be analyzed, a polarizer, the axis of polarization of which is perpendicular to the axis of polarization of the emitted laser beam, a means for sensing the transmitted laser beam, and for converting the latter into an electrical signal, and electronic processing means for separating this electrical signal into two components and for computing, from these components, the index I representing the formation index. The formation index I is defined as follows: ##EQU1## where RMS commonly designates the effective variable value of the laser beam transmitted through the sheet, commonly referred to as the formation component, and the value M is the mean value of the transmitted signal, commonly referred to as the basis weight component.
The formation sensor generally comprises a U-shaped chassis between the branches of which the sheet of paper to be analyzed passes continuously and at constant speed. The upper branch of the U contains the laser, of the He-Ne type, and the lower branch the polarizer and the system for receiving the transmitted laser beam.
In order to calibrate such a device, it is necessary to have a standard available which produces a modulation of the optical signal which is detectable by the system for receiving the transmitted laser beam.
The calibration process is essential for at least two reasons:
1. the paper-making industry is moving toward requiring objective measurement of differences in paper characteristics which, until the appearance on the market of devices of the present type, were very subjective, being determined by visual examination of the transparency of the sheet of paper.
2. the possible appearance of drifts in the results of the measurements made by the formation sensor, due to inherent aging of the electronic system and the optical components of the device.